The present invention relates to an ultraviolet curing resin composition for optical disks.
An optical disk such as CD, CD-R and DVD records information on a transparent substrate thereof such as polycarbonate by pits and lands and forms an information recording layer by providing a reflective film that allows laser beam to bounce off of the surface of pits and lands for reading out the information so as to have one or a plurality of such information recording layers. In optical disks, aluminum, gold, silicon, silicon compounds such as SiC and SiN, or the like have been conventionally formed into thin films by sputtering for being used as the reflective film. Also, in an optical disk of a type such as to have a plurality of information recording layers and read out information in these information recording layers from one side of the optical disk, at least one of reflective layers is formed as a translucent reflective film.
On the other hand, in the optical disk, an ultraviolet curing resin composition is used as a role of protecting the information recording layers for the purpose of performing surface protective coating on the above-mentioned reflective film. Also, in an optical disk of a higher recording density type such that two sheets of substrates are stuck to each other on backsides thereof, namely, so that transparent substrates thereof face outward, an ultraviolet curing resin composition is used for sticking the substrates together.
In the above-mentioned reflective film, thin films made of gold, silicon and silicon compounds have been used for translucent films of DVD requiring the compatibility between high transmittance and reflectance, the problem is that gold costs high while silicon and silicon compounds are troublesome to handle due to easy breaking and cracking and are oxidized in being formed into films so as to contaminate a substrate surface of an optical disk. Accordingly, the use of thin films made of silver or silver alloy in which a slight quantity of gold, palladium, copper and the like are added to silver has been studied as substitutes for the above-mentioned thin films. For example, a reflective film for optical disks comprising silver alloy comprising silver as a major component is disclosed in U.S. Pat. No. 6,007,889.
Silver or the above-mentioned silver alloy, however, has a problem of being essentially inferior in light resistance and not having a sufficient durability due to easy corroding under the conditions of high temperature and high humidity, so that thin films made of silver or the silver alloy are used with difficulty as a reflective film for forming an information recording layer. In order to improve this, for example, a technique is disclosed in Japanese Unexamined Patent Publication No. 2001-167478 for maintaining initial properties by using an adhesive comprising an ultraviolet curing type composition with low water absorption and low moisture permeability for sticking the substrates together.
Conventional techniques, however, offer no solution to a problem such that silver and silver alloy are inferior in light resistance and they also do not necessarily have sufficient adhesive force.
In view of the above-mentioned existing situation, the present invention is intended for providing an ultraviolet curing resin composition such as to improve the light resistance of thin films made of silver or silver alloy and protect them against corroding and perform a high adhesive strength, in an optical disk in which an information recording layer is formed by the thin films made of silver or silver alloy.
The present invention is an ultraviolet curing resin composition for an optical disk in which an information recording layer is formed by thin films made of silver or alloy comprising silver as a major component, the ultraviolet curing resin composition for an optical disk comprising (1) a (meth)acrylate compound having a number average molecular weight of 700 to 3000 derived from epoxy resin, (2) a (meth)acrylate monomer having one or more of unsaturated ethylene groups, and (3) a polymerization initiator represented in the following general formula (I). 
In the formula, R1 denotes an alkyl group with a carbon number of 1 to 4. R2 denotes a hydrogen atom or an alkyl group with a carbon number of 1 to 4. R3 denotes a phenyl group or an alkyl group with a carbon number of 1 to 4.
In the present invention, the above-mentioned (meth)acrylate compound (1) having a number average molecular weight of 700 to 3000 derived from epoxy resin may be an adduct of xcex5-caprolactone-modified (meth)acrylate to epichlorohydrin-bisphenol A type (epi-bis type) epoxy resin. Also, in the present invention, a tetrazole derivative may be additionally mixed. The present invention is hereinafter detailed.